IT help: pc not turning on

Odd one: turn the pc off at the mains and leave it for a while. When I turn the mains back on the computer will not power up. However, if I leave it for 15-30 mins it starts up normally. Anyone know what might be causing this?

There's a 'power ok' line that comes out of the psu which the motherboard checks before starting up. The psu could be on the way out hence sometimes it isn't 'ok'. PSU's are the most unreliable bit of a pc in my experiance (had one fail over christmas), but cheap and quick to fix.